Common Attack Pattern Enumeration and Classification
A Community Resource for Identifying and Understanding Attacks
HTTP Request Splitting (also known as HTTP Request Smuggling) is an attack pattern where an attacker attempts to insert additional HTTP requests in the body of the original (enveloping) HTTP request in such a way that the browser interprets it as one request but the web server interprets it as two.
There are several ways to perform HTTP request splitting attacks. One way is to include double Content-Length headers in the request to exploit the fact that the devices parsing the request may each use a different header. Another way is to submit an HTTP request with a "Transfer Encoding: chunked" in the request header set with setRequestHeader to allow a payload in the HTTP Request that can be considered as another HTTP Request by a subsequent parsing entity. A third way is to use the "Double CR in an HTTP header" technique. There are also a few less general techniques targeting specific parsing vulnerabilities in certain web servers.
Microsoft Internet Explorer versions 5.01 SP4 and prior, 6.0 SP2 and prior, and 7.0 contain a vulnerability that could allow an unauthenticated, remote attacker to conduct HTTP request splitting and smuggling attacks.
The vulnerability is due to an input validation error in the browser that allows attackers to manipulate certain headers to expose the browser to HTTP request splitting and smuggling attacks. Attacks may include cross-site scripting, proxy cache poisoning, and session fixation. In certain instances, an exploit could allow the attacker to bypass web application firewalls or other filtering devices.
Microsoft has confirmed the vulnerability and released software updates
Skill or Knowledge Level: Medium
Good understanding of the HTTP protocol and the parsing mechanisms employed by various web servers
Make sure to install the latest vendor security patches available for the web server.
If possible, make use of SSL.
Install a web application firewall that has been secured against HTTP Request Splitting
Use web servers that employ a tight HTTP parsing process
System integration testing must include security checks to protect against Multiple Interpretation Errors across systems.
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